This research will examine cell interactions involved in two fundamental events in nervous system development--the assignment of a neuronal phenotype to cells and the directed growth of their axons. We will study these processes in the grasshopper embryo whose excellent experimental properties permit a detailed analysis at the cellular and molecular level in an in vivo preparation. We will provide a background for the experiments by describing in detail, using histological and immunocytochemical techniques, how and when the components of the retina and lamina of the visual system develop. We will examine the importance of intercellular communication for the development of retina from epidermal cells and for the triggering of interneuron differentiation in the lamina, using dye injection and cell elimination. We will describe the substrates used for retina axon outgrowth both during the initial establishment of connections between the retina and lamina and during subsequent development, paying particular attention to the role of the basement membrane. We will examine the basement membrane using antibodies and lectins and evaluate its possible role in axon guidance, which will be experimentally tested. This research is valuable because the simple organization and experimental suitability of the grasshopper embryo permits carefully controlled experiments that are impossible on more complex nervous systems. The results should increase our understanding of nervous system development in general and may allow the formation of principles and tests that can be used to study more complex nervous systems.